During operation of a semiconductor laser device, i.e., when it is generating optical radiation, it also generates relatively large amounts of heat that must be quickly and efficiently conducted away from the device, lest its temperature rise to an undesirable level and its useful device lifetime be undesirably reduced.
U.S. Pat. No. 4,772,935 teaches a process for thermal-compression bonding of a silicon integrated circuit due to a package comprising a header. The process utilizes sequential formation on a major surface of a silicon wafer (substrate), prior to its being cut into a plurality of dies, the following successive layers: (1) an adhesion layer of titanium; (2) a barrier layer, preferably of tungsten; (3) a bonding layer, preferably of gold. Also, a stress-relieving layer, preferably of gold, can be formed earlier between the adhesion layer of tungsten and the major surface of the wafer. Thereafter, the substrate is cut into the plurality of dies, each of which is bonded, for example, to a ceramic header ("submount"). Prior to the die's being bonded to the header, a major surface of the header is coated with a layer of gold that, in turn, is coated with a binding layer of solder, preferably a gold-tin eutectic. The purpose of the adhesion layer is to promote adhesion of the tungsten barrier layer to the substrate. The purpose of the barrier layer is to suppress migration of silicon from the substrate into the originally eutectic binding layer of solder, such migration causing an undesirable increase in the melting temperature of the binding layer of solder and hence a required undesirably high temperature rise in the wafer during the thermal-compression bonding process in which the binding layer of solder must be melted to wet the surface to be bonded. The purpose of the bonding layer of gold is to protect the barrier layer from oxidation that would weaken the resulting bond.
In the aforementioned patent, the thickness of the binding layer of solder was reported to be 0.5 to 1.0 mil, or 12.7 to 25.4 .mu.m. Such a relatively large thickness, regardless of relatively small thicknesses of other layers, is not desirable in the context of relatively high power (over 100 milliwatt) lasers, because of the need of a significantly higher thermal conductance and hence significantly smaller thickness of the entire resulting bond between laser and submount.
On the other hand, when a gold-tin solder layer is made desirably thin from the standpoint of good and sufficient thermal conductance--i.e., about 4 .mu.m or less--then surface regions of the solder suffer from premature freezing (solidification) during the bonding process when heat is applied in quantities sufficient to raise the temperature of the solder above its melting temperature, namely above 280.degree. C. in cases where the gold-tin solder has a eutectic composition (gold: 80 per centum by weight; tin: 20 per centum by weight) and hence a desirable minimum melting temperature. This premature freezing is caused by migration of tin away from the surface of the solder (initially having a eutectic or even a tin-rich nearly eutectic composition), whereby (because the solder's surface regions no longer have the eutectic or nearly eutectic compositions) the melting (=freezing) point of the solder's surface regions dramatically increases, namely, by about 30.degree. C. per centum (by weight) decrease in the tin component in the neighborhood of the eutectic composition on its tin-poor side. Consequently, surface regions of the solder undesirably solidify ("freeze" ) during bonding, because the bonding process cannot be performed at a temperature that is high enough to prevent this freezing and at the same time is low enough so as not to injure the device. This premature freezing of the solder causes poor "wetting" of the surface of the gold bonding layer (on the barrier layer) on the device by the solder and consequently poor bonding of the device to the submount. Thus, during subsequent device operation, the resulting poor thermal conductance of the resulting bond (caused by the poor "wetting") tends to cause injurious overheating of the device, and the resulting poor mechanical adhesion property of the bond tends to allow the device to detach from the submount.
On the other hand, gold-tin solder is preferred over other solders--such as indium, lead-tin, tin-lead, tin-indium, lead-indium-silver--because of its relatively high Young's modulus of elasticity and hence its resulting mechanically more stable (rigid) bond. Also bonds made from gold-tin solder tend to have desirably lower creep, such lower creep being associated with the relatively high melting temperature (280.degree. C. or more) of a gold-tin solder. Moreover, gold-indium solder has too high a melting temperature (about 457.degree. C.) for bonding lasers, because such a high temperature tends to injure the laser during bonding.
Therefore, it would be desirable to have a bonding method using a gold-tin solder, but which does not suffer from the short-comings of prior art.